Games : Does Garry's Mod download addons twice? If not, what is it downloading?

You can find a copy of this question asked (by me) on the Facepunch Forums.

When I subscribe to an addon on the Garry's Mod workshop, Steam instantly downloads them in the download manager.

When I open Garry's Mod, it seems to be downloading even more things, and both stages uses bandwidth as far as I can see.

As in: Garry's Mod does not use the files downloaded by Steam. Does it?

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My Questions:

1. When Steam downloads an item from the workshop, where is it stored? In the addons folder or elsewhere?


2. Why is Garry's Mod downloading my addons again? (And is it downloading it twice or only updates?) I've noticed that both steps involve using a lot of bandwidth...


3. If the addons downloaded by Steam are stored elsewhere, is there any way to make Garry's Mod copy-paste instead of downloading them again (if it is doing that)?